High-strength copper alloys with excellent bending workability and terminal connectors using the same

ABSTRACT

The invention aims at providing high-strength copper alloy, especially phosphor bronze, with excellent bending workability. The excellently bendable high-strength copper alloy is obtained through grain size control whereby a finally cold rolled copper alloy with a tensile strength and 0.2% yield strength different by not more than 80 MPa is allowed to have characteristics such that its mean grain size (mGS) after annealing at 425° C. for 10,000 seconds is not more than 5 μm and the standard deviation of the mean grain size (σGS) is not more than ⅓×mGS. Improvements in characteristics presumably attributable to the synergistic effect of grain-boundary strengthening and dislocation strengthening are stably achieved by the adjustments of cold rolling and annealing conditions and by the study of the correlation between pertinent characteristic values after the final rolling. The method of processing the alloy comprises cold rolling to a reduction percentage of at least 45%, final annealing to the extent that the mean grain size (mGS) is not more than 3 μm and the standard deviation of the mean grain size (σGS) is not more than 2 μm, and final cold rolling to a reduction percentage of 10-45%.

FIELD OF THE INVENTION

This invention relates to high-strength copper alloys, especially high-strength phosphor bronze, having excellent bending workability for use in electronic parts such as terminal connectors, a method of manufacturing the same, and terminal connectors using the same.

PRIOR ART

Narrow strips of phosphor bronze such as C5210 and C5191 (in conformity with JIS H 3110 and JIS H 3130, respectively) and copper alloy materials such as C2600 (JIS H 3100) that have outstanding workability and mechanical strength are widely employed for such uses as electronic parts like terminal connectors.

Recent years have witnessed stronger tendencies toward slimmer and smaller electronic parts than ever, and accordingly there have been demand for thinner strips of copper alloy as materials for those parts. Thinner materials are required to possess, in themselves, sufficient strength to maintain the contact pressure and other forces needed of the resulting connectors and the like. Meanwhile, the manufacture of tinier electronic parts calls for material of adequately high bendability to permit bending to smaller bend radii than heretofore so that the parts can fulfill their functions within narrower spaces. Thus, the materials are required to have contradictory properties of high strength and good bending workability.

In attempts to meet the demand, high-strength copper alloys such as beryllium copper and titanium copper and, where electric conductivity is an additional requirement, Corson (Cu—Ni—Si) alloy and chromium-copper (Cu—Cr, Cu—Cr—Zr, Cu—Cr—Sn, etc.) alloys have come into use.

However, those high-strength copper alloys that are comparatively new varieties of copper alloys for electronic parts have limitations in the demand and supply and distribution in the market. For example, difficulties are involved in their extensive acceptance in the market where more and more weight is being placed on global standards. Another factor that hinders wide-spread adoption of those high-strength copper alloys is that they are costlier than ordinary phosphor bronze and other existing copper alloys.

In view of the foregoing, there is demand for further improvements in strength and workability of the conventional copper alloys such as brass and phosphor bronze that have been deemed to have relatively great mechanical strength among ordinary copper alloys. As for the workability, good bending property is desired in particular. This is because more and more severe bending is involved in the fabrication of terminal connectors, lead frames, and other metallic members of electronic components to keep pace with the progress of higher density packaging in the fields of PDA (Personal Digital Assistance), digital cameras, and video cameras.

In general, attempts to enhance the strength of metals depend on combinations of solid-solution, precipitation, grain-boundary, dislocation and other hardening or strengthening techniques. Phosphor bronze whose compositional ranges are standardized is a copper alloy of the solid-solution strengthening type. Efforts have been made to improve its strength by proper conditioning such as cold rolling and annealing from the viewpoints of intergranular strengthening and dislocation hardening. Actual developments, however, are behind the rapid progress of demand for lighter, thinner, and smaller electronic parts in recent years.

(Problem that the Invention is to Solve)

Under such circumstances, the problem that the invention is to solve is to develop a technique whereby high strength and bendability are combinedly imparted to solid-solution strengthened type copper alloys, especially to general-purpose phosphor bronze.

(Means of Solving the Problem)

Copper alloys of the solid-solution strengthened type, especially general-purpose phosphor bronze, when further strengthened by grain-boundary and dislocation techniques, i.e., by heat treatment and rolling, give final products that are unable to observe clearly its grain boundaries. In other words, as deformation of a metal strip due to cold working proceeds, variations of local transgranular deformation become increasingly conspicuous, giving birth to many different deformation bands such as shear bands and micro-bands. These deformation bands make the grain boundaries that had been formed by recrystallization before cold working discontinuous, and, when the cross section is etched and then observed under an optical microscope, the crystal structure looks indistinct. Inspection of the structure by a transmission electron microscope shows that the structure, even after cold working to a reduction ratio of about 20%, retains part of the pre-working recrystallization grain boundaries. It is already covered with a cell structure, which hampers precise determination of the grain size. This has been a major obstacle to improvements in the properties of cold rolled materials.

The present inventors have adjusted the conditions for cold rolling and annealing of phosphor bronze and investigated the correlations among various property values after final rolling. As a result, they have succeeded in steady improvements in the properties presumably owing to the composite effects of grain-boundary strengthening and dislocation strengthening. The present invention provides high-strength copper alloys with excellent bending workability that can be defined as follows:

(1) A high-strength copper alloy having excellent bending workability characterized in that it is a finally cold rolled copper alloy with a tensile strength and 0.2% yield strength different by not more than 80 MPa, the alloy having characteristics such that the mean grain size (mGS) thereof after annealing at 425° C. for 10, 000 seconds is not more than 5 μm and the standard deviation of the mean grain size (σGS) is not more than ⅓×mGS.

(2) A high-strength copper alloy having excellent bending workability according to (1) characterized by comprising from 1 to 11 mass % Sn, from 0.03 to 0.35 mass % P, and the balance Cu and unavoidable impurities, with a tensile strength termed TS_(Sn) (MPa) being TS_(Sn)>500+15×Sn (Sn: tin concentration (mass %)), the alloy having characteristics such that the mean grain size (mGS) thereof after annealing at 425° C. for 10,000 seconds is not more than 5 μm and the standard deviation of the mean grain size (σGS) is not more than ⅓×mGS.

(3) A high-strength copper alloy having excellent bending workability according to (1) or (2) characterized by comprising from 1 to 11 mass % Sn, from 0.03 to 0.35 mass % P, and the balance Cu and unavoidable impurities, the alloy having characteristics such that the mean grain size (mGS (μm)) thereof after annealing at 425° C. for 10,000 seconds is mGS<2.7×exp (0.0436×Sn (Sn: tin concentration (mass %)).

(4) A high-strength copper alloy having excellent bending workability according to (1), (2) or (3) characterized by being a phosphor bronze which comprises from 1 to 11 mass % Sn, from 0.03 to 0.35 mass % P, from 0.05 to 2.0 mass %, in total, of one, two, or more selected from among Fe, Ni, Mg, Si, Zn, Cr, Ti, Zr, Nb, Al, Ag, Be, Ca, Y, Mn, and In, and the balance Cu and unavoidable impurities.

(5) A high-strength copper alloy having excellent bending workability according to (1), (2) or (3) characterized by being a phosphor bronze which comprises from 1 to 11 mass % Sn, from 0.03 to 0.35 mass % P, from 0.05 to 2.0 mass %, in total, of one, two, or more selected from among Fe, Ni, Mg, Si, Zn, Cr, Ti, Zr, Nb, Al, Ag, Be, Ca, Y, Mn, and In, and the balance Cu and unavoidable impurities, with particles that mainly consist of precipitation or crystallization products of the alloying metals, 0.1 μm or more in diameter, being present in a number of not fewer than 100 per square millimeter of a cross section cut in parallel to the rolling direction.

The invention also provides methods of manufacturing high-strength copper alloys having excellent bending workability, under the conditions defined below.

(6) A method of manufacturing high-strength copper alloy having excellent bending workability characterized by the steps of cold rolling to a reduction ratio of at least 45%, final annealing to the extent that the mean grain size (mGS) is not more than 3 μm and the standard deviation of the mean grain size (σGS) is not more than 2 μm, and final cold rolling to a reduction ratio of from 10 to 45%.

(7) A method of manufacturing high-strength copper alloy having excellent bending workability characterized by the steps of cold rolling to a reduction ratio of at least 45%, final annealing to the extent that the mean grain size (mGS) is not more than 2 μm and the standard deviation of the mean grain size (σGS) is not more than 1 μm, and final cold rolling to a reduction ratio of from 20 to 70%.

(8) A method of manufacturing high-strength copper alloy having excellent bending workability according to (6) or (7) characterized by stress relief annealing of the cold rolled material that has been finally cold rolled to a reduction ratio X (%) and has a tensile strength of TS₀ (MPa), until the tensile strength TS_(a) (MPa) after the annealing is TS_(a)<TS₀−X.

The methods (6) to (8) are applicable to the manufacture of the copper alloys defined in (1) to (5). The invention further provides methods of manufacturing high-strength copper alloys having excellent bending workability, under the conditions defined below.

(9) A method of manufacturing high-strength copper alloy having excellent bending workability defined in any of (1) to (5) characterized by the steps of cold rolling to a reduction ratio of at least 45%, final annealing to the extent that the mean grain size (mGS) is not more than 3 μm and the standard deviation of the mean grain size (σGS) is not more than 2 μm, and final cold rolling to a reduction ratio of from 10 to 45%.

(10) A method of manufacturing high-strength copper alloy having excellent bending workability defined in any of (1) to (5) characterized by the steps of cold rolling to a reduction ratio of at least 45%, final annealing to the extent that the mean grain size (mGS) is not more than 2 μm and the standard deviation of the mean grain size (σGS) is not more than 1 μm, and final cold rolling to a reduction ratio of from 20 to 70%.

(11) A method of manufacturing high-strength copper alloy having excellent bending workability defined in any of (1) to (5) characterized by the stress relief annealing, in relation to (9) or (10), of a cold rolled material that has been finally cold rolled to a reduction ratio X (%) and has a tensile strength of TS₀ (MPa), until the tensile strength TS_(a) (MPa) is TS_(a)<TS₀−X.

As for the applications, the invention provides:

(12) Terminal connectors using the high-strength copper alloys having excellent bending workability according to any of (1) to (5).

MODES OF EMBODYING THE INVENTION

The grounds on which the various elements that constitute the present invention are restricted will now be explained for individually claimed inventions (called also collectively the present invention).

(The Invention of High-Strength Copper Alloy Having Excellent Bending Workability According to (1) Above)

The invention according to (1) above defines that a high-strength copper alloy having excellent bending workability with a difference between tensile strength and 0.2% yield strength being not more than 80 MPa, has properties such that the mean grain size (mGS) thereof after annealing at 425° C. for 10,000 seconds is not more than 5 μm and the standard deviation of the mean grain size (σGS) is not more than ⅓ mGS.

For the purposes of the invention the grain size is determined by the cutting method in conformity with the procedure specified in JIS H 0501 (JIS stands for Japanese Industrial Standards). To be more concrete, the number of the grains completely sectioned along a predetermined length of segment is counted, and the mean value of the cut lengths is measured as the grain size. The standard deviation that characterizes the dispersion does not represent a standard deviation of the cut lengths but a standard deviation of the grain sizes.

The copper alloy according to the present invention is obtained as an end product, basically, by a method which comprises cold rolling of the alloy material to a reduction ratio of at least 45%, and thereafter either final annealing to the extent that the mean grain size (mGS) is not more than 3 μm and the standard deviation of the mean grain size (σGS) is not more than 2 μm, and final cold rolling to a reduction ratio of from 10 to 45% or, alternatively, final annealing to the extent that the mean grain size (mGS) is not more than 2 μm and the standard deviation of the mean grain size (σGS) is not more than 1 μm, and final cold rolling to a reduction ratio of from 20 to 70%. As noted already, addition of strength by grain-boundary and dislocation strengthening techniques, i.e., by heat treatment and rolling, makes it impossible for the end product to observe clearly its grain boundaries. Stated differently, as deformation of a metal strip by cold working proceeds, variations in local transgranular deformation grains become so conspicuous that many different deformation bands such as shear bands and micro-bands result. These deformation bands make the grain boundaries that had been formed by recrystallization before cold working discontinuous, and, when the cross section is etched and then observed under an optical microscope, the crystal structure looks indistinct. Even after cold working to a ratio of about 20%, the structure inspected under a transmission electron microscope shows that the structure retains part of the pre-working recrystallization grain boundaries. It is already covered with a cell structure, which hampers precise determination of the grain size. Thus, precise determination of the grain size has been extremely difficult.

It has now been found that there is a correlation between the recrystallization behavior of a copper alloy after cold working and the properties of the alloy that combines bending workability and strength. The correlation is helpful in identifying the material. Thus, the present invention provides a copper alloy which combines excellent bending workability with high strength, with a tensile strength and 0.2% yield strength different by not more than 80 MPa, the alloy having characteristics such that the mean grain size (mGS) thereof after annealing at 425° C. for 10,000 seconds is not more than 5 μm and the standard deviation of the mean grain size (σGS) is not more than ⅓ mGS.

When a metal material is annealed and cold worked, it is common that as the degree of cold rolling increases the difference between the tensile strength and the 0.2% yield strength decreases. Simultaneously the ductility decreases, making the metal susceptible to cracking upon bending. It is now found under the invention that the decrease in ductility can be minimized by adjusting the conditions of final annealing before the final rolling as well as the conditions of the preceding cold working. This characteristic promises remarkably beneficial effect upon a high-strength copper alloy having a property such that the difference between the tensile strength and 0.2% yield strength is not more than 80 MPa.

The copper alloy according to the invention is defined also by its unique property that its mean grain size is maintained below 5 μm when annealed under the condition of 425° C. for 10,000 seconds, which condition allows considerable growth of grains in ordinary copper alloys. The copper alloy of the invention, obtained as an end product by either final annealing to the extent that the mean grain size (mGS) is not more than 3 μm and the standard deviation of the mean grain size (σGS) is not more than 2 μm, and final cold rolling to a reduction ratio of from 10 to 45% or, alternatively, final annealing to the extent that the mean grain size (mGS) is not more than 2 μm and the standard deviation of the mean grain size (σGS) is not more than 1 μm, and final cold rolling to a reduction ratio of from 20 to 70%, possesses an ultrafine grains that cannot exhibit the crystal boundaries in the end product. The ultrafine grains have the unique character of maintaining a mean grain size of not more than 5 μm, without grain growth, upon annealing under the conditions of 425° C. for 10,000 seconds. By utilizing this character, the copper alloy of the invention can be distinguished from other copper alloys and defined as such.

Products of copper alloys according to the present invention undergo little decreases in ductility upon final cold working in the process of the products, and they combine high strength with excellent bending workability.

The mean grain size (mGS) of the metal after annealing at 425° C. for 10,000 seconds is preferably not more than 3 μm, since it improves the relation between the tensile strength and bending workability.

Even if the mean grain size (mGS) is not more than 5 μm, its beneficial effect is limited if there is a scatter of the size. As will be described later, the process of products must be strictly controlled to obtain a homogeneous fine grains. The over-all tolerance of the scatter, in terms of the standard deviation of the grain size, should be not more than ⅓ mGS, because a standard deviation (σGS) in excess of ⅓ mGS reduces the improvement upon the bending workability.

(The Invention of High-Strength Copper Alloy Having Excellent Bending Workability According to (2) Above)

This invention restricts the inventive copper alloys to phosphor bronze having a high tensile strength.

Unlike other copper alloys, phosphor bronze that contains tin as a solid-solution strengthening element, varies in the work-hardening property with its tin concentration. With this in view, the invention specifically defines the effective range for a high-strength material, in terms of an empirically obtained relation between the tin concentration and tensile strength, as:

Tensile strength TS_(Sn)(MPa)>500+15×Sn (tin conc., mass %)

The better the actual values satisfy the above relation the elements referred to in (1) above will be the more effective. In other words, where the reduction ratio of cold working is low, the decrease of ductility is limited, favorable bending workability is retained without the need of controlling the grain size, and the influences of the process conditions prior to the final annealing are reduced.

(The Invention of High-Strength Copper Alloy Having Excellent Bending Workability According to (3) Above)

This invention again restricts the inventive copper alloys to phosphor bronze and defines the relation between the mean grain size (mGS: μm) after annealing at 425° C. for 10,000 seconds and the tin concentration (Sn: mass %) to be:

mGS<2.7×exp(0.0436×Sn).

Phosphor bronze exhibits a grain growth behavior peculiar to itself. Thus, it is desirable that the grains should be adjusted so that the mean grain size after the above annealing may be mGS<2.7×exp(0.0436×Sn). This is a formula empirically found from the correlation among the working conditions, properties (strength and bending workability), and the grain size after heat treatment at 425° C. for 10,000 seconds, of a phosphor bronze containing from 1 to 11%, preferably from 2 to 10%, tin. If the mGS is more than the level specified above, the effect of grain-refining is negligible, no appreciable increase in strength is possible without an increase in the rolling reduction ratio, ductility of highly strengthened material decrease, and the bending workability remains unimproved.

With regard to the relation between the grain size and strength (yield strength), the major effect of basic importance is that of grain refinement commonly represented by the Hall-Petch equation. On this basis it has been found that the grain size after recrystallization can subsequently increase the work-hardening ability itself.

For the purpose of the practical use of phosphor bronze, the above feature permits high strengthening by rolling at a low reduction ratio. While the lower limit is not definitely specified, it should be noted that if the mean grain size (mGS) after the final annealing is as small as below 0.4 μm, the ductility once lowered by the cold rolling before the final annealing is not fully recovered; rather, the ductility further drops as a result of the final cold rolling. For this reason it is desirable that the mGS should be not less than 0.4 μm.

(The Invention of High-Strength Copper Alloy Having Excellent Bending Workability According to (4) Above)

This invention adds from 0.05 to 2.0 mass %, in total, of one, two, or more selected from among Fe, Ni, Mg, Si, Zn, Cr, Ti, Zr, Nb, Al, Ag, Be, Ca, Y, Mn, and In to the copper alloy, especially phosphor bronze, specified above.

The grounds for the addition of Fe, Ni, Mg, Si, or/and Zn will first be explained.

Trace addition of Fe, Ni, Mg, or/and Si to phosphor bronze as a copper alloy results in the formation of intermetallic compounds between those elements and P. The compounds so formed are dispersed in the matrix to improve the properties of the phosphor bronze made primarily by grain-boundary strengthening and solid-solution strengthening in accordance with any of (1) to (3) above. Of these combinations, Fe—P or the like, for example, maybe chosen to form an intermetallic compound by precipitation. Its dispersion not only adds strength by precipitation strengthening of the resulting alloy itself but also effectively helps pinning of the grain boundaries by means of the residual particulates of the precipitation and crystallization products. In addition, it slows down the growth of grains and facilitates the grain refinement. For these purposes a total amount of 0.05 mass % is necessary but an addition of more than 2.0 mass % is rather detrimental to electric conductivity and other properties.

Zn is an element which, when added to a copper alloy, improves the thermal peeling resistance of tin and solder plates from the alloy surface. It improves effectively when added in an amount of about 0.1 mass % or more, but the addition of more than 0.5 mass % saturates the beneficial effect and lowers the electric conductivity.

As described above, Fe, Ni, Mg, Si, and Zn are the elements that add strength of phosphor bronze or improve the thermal peeling resistance of tin and solder plates on the alloy, and their addition is recommended. The amount to be added is decided in consideration of the bending workability and electric conductivity of the resulting alloy and ranges, in all, from 0.05 to 2.0 mass %. The reasons are that a total amount of less than 0.05 mass % is not large enough to improve the strength or enhance the thermal peeling resistance whereas an amount of more than 2.0 mass % deteriorates the bending workability and reduces the electric conductivity. The reduction of electric conductivity is particularly profound with the low-tin, high-conductivity phosphor bronze having a tin concentration of about 1 to 4 mass %. Of those addition elements, Zn desirably ranges in amount from 0.1 to 0.5 mass % for the reason stated above.

The addition of the elements other than those mentioned above, i.e., Cr, Ti, Zr, Nb, Al, Ag, Be, Ca, Y, Mn, and In will now be explained.

These elements enhance the strength of copper alloy by solid-solution strengthening and precipitation strengthening. As with Fe, Ni, Mg, Si, and Zn described above, such an element or elements are added in an amount, in all, of not more than 1.0 mass % for a further increase in the strength of the resulting alloy.

Thus the strength of an alloy is improved by the addition of from 0.05 to 2.0 mass %, in total, of one, two, or more selected from among Fe, Ni, Mg, Si, Zn, Cr, Ti, Zr, Nb, Al, Ag, Be, Ca, Y, Mn, and In.

The addition elements mentioned immediately above are typical elements useful from the economic viewpoint too. Copper alloys that contain as auxiliary constituents any other element or elements primarily capable of solid-solution strengthening without the deterioration of the conductivity and other properties of the alloy also come within the scope of the present invention.

(The Invention of High-Strength Copper Alloy Having Excellent Bending Workability According to (5) Above)

This invention defines the distribution of the precipitation or crystallization product of the alloying elements in the invention as defined in (4) above.

With the aim of grain refinement, the optimum state peculiar to phosphor bronze has now been found out. Presumably closely related to the intergranular energy and the like of phosphor bronze, particles ranging in diameter from 0.1 μm to 10 μm present at the rate of at least 100 particles per square millimeter as counted by observation of a cross section, prove remarkably effective in grain refinement. The particles are coarse particles of precipitation or crystallization product, and regardless of the composition of the precipitation or crystallization product, the particles are found to have a grain-refining effect.

In the process of grain refinement it is presumed that the particles that actually contribute to the nucleation of grains and grain boundary pinning include finer particles. So far as the inspection under a scanning electron microscope is concerned, an outstanding grain-refining effect is observed in the cross sectional structure with the above-mentioned particle distribution. Thus, as a substitutional characteristic of grain refinement, the distribution of the precipitation or crystallization product is set forth.

(The Invention of Method of Manufacturing High-Strength Copper Alloy Having Excellent Bending Workability According to (6) Above)

This invention relates to a method of manufacturing high-strength copper alloy having excellent bending workability. In particular, it relates to a method of manufacturing high-strength copper alloy having excellent bending workability by the repetition of cold rolling and annealing in specified steps of final cold rolling, final annealing prior to the cold rolling, and the cold rolling even before the final annealing.

Subsequently this invention too is basically aimed at achieving the effect of grain refinement before the final rolling that follows the final annealing. Assuming that the thickness of the material before the cold rolling is t₀ and the thickness after the cold rolling is t, the reduction ratio X of the cold rolling before the final annealing is defined as

X=(t ₀ −t)/t ₀×100(%).

The reduction ratio is then specified to be not less than 45%. This is because a ratio below 45% will scarcely refine the grain size after the final annealing, despite adjustments of the heat treatment conditions for the final annealing.

The mean grain size after the annealing is specified to be not more than 3 μm and the standard deviation of the mean grain size is specified to be not more than 2 μm. The ground for these limits is that a homogeneous fine-grain structure must be obtained through precise control of the heating temperature profile during annealing.

The term fine recrystallized grain as used herein means that, when the mean grain size (mGS) is 3 μm and the standard deviation (σGS) is 2 μm, not less than 99% of the diameters of the individual crystal grains is mGS+3σGS, or not more than 9 μm, although the distribution of grain size is not normal distribution.

Existence of grains 8 μm or more in size in a recrystallized structure is often undesirable, and therefore it is desired that the standard deviation of grain size is not more than 1.5 μm.

The influences of the reduction ratio of cold rolling before the final annealing upon the recrystallized grains after the final annealing are such that the higher the reduction ratio the finer the grain size of the recrystallized grain after the annealing. At the same time, nucleation of grains and the subsequent secondary recrystallization behavior tend to large dispersion, and a duplex grain structure is likely to develop.

Above all, the copper alloy having a pure copper type recrystallization structure with a high copper concentration shows a strong tendency.

On the other hand, brass containing more than 30 mass % Zn and phosphor bronze containing more than 4 mass % Sn are relatively easy to regularize the size of the recrystallized grains after high reduction working.

It is necessary, in view of the foregoing, to optimize the annealing conditions, i.e., temperature, time, and temperature profile, for each alloy to obtain a recrystallized structure.

If either the mean grain size or the standard deviation is outside the range specified, i.e., not more than 3 μm or not more than 2 μm, respectively, the ability of high work hardening upon the final cold rolling is not obtained.

Final cold rolling of an alloy having a mean grain size of not more than 3 μm and a standard deviation of not more than 2 μm to a reduction ratio of 10 to 45%, gives a copper alloy with high strength and excellent bending workability.

A reduction ratio of less than 10% is limited in grain-refining effect and imparting good bending workability for ordinary copper alloys having a mean grain size of about 10 μm after the final annealing. On the other hand, a reduction ratio of more than 45% decreases the bending workability and narrows the application range of the alloy as a material for contacts and other metal components worked by bending.

(The Invention of Method of Manufacturing High-Strength Copper Alloy Having Excellent Bending Workability According to (7) Above)

This invention sets forth that the mean grain size is not more than 2 μm and the standard deviation of the mean grain size is not more than 1 μm, thus narrowing the scatter of the grain size, i.e., a standard deviation of not more than 2 μm, specified in the invention as defined in (6) above. The consequent effect of uniform grain refinement permits a further increase in the reduction ratio of the final cold rolling to 20 to 70%, which makes it possible to obtain a high-strength copper alloy without a deteriorating of the bending workability.

(The Invention of Method of Manufacturing High-Strength Copper Alloy Having Excellent Bending Workability According to (8) Above)

This invention defines the amount of decrease in the tensile strength of the above-specified copper alloy upon stress relief annealing after the final rolling. According to the definition, the tensile strength before the stress relief annealing is TS₀ (MPa), the tensile strength after the stress relief annealing is TS_(a) (MPa), and TS_(a)<TS₀−X (the ratio of reduction (%) by the final cold rolling).

Phosphor bronze, nickel silver and the like are sometimes annealed for stress relief. Unlike the recrystallization annealing that is conducted prior to the final rolling, stress relief annealing is aimed at recovering the ductility (workability) after cold rolling and also improving the springiness and other properties. For these purposes it is commonly performed with copper alloys such as phosphor bronze for spring use (C5210: JIS H 3130).

Stress relief annealing may be done, as needed, by a tension annealing line or the like after the final rolling.

The copper alloys according to the present invention, even after the stress relief annealing, is superior in strength and bending workability than the alloys made by prior art methods.

When an annealed material of a particularly fine grain size is to be cold rolled, it is effective to conduct the stress relief annealing corresponding to the final reduction ratio so as to minimize the loss of the ductility. Where bending workability is to be enhanced in particular, the stress relief annealing is done under conditions such that, assuming that the reduction ratio of the final cold rolling is X % and the cold rolled material has a tensile strength (TS₀:MPa), the tensile strength of the cold rolled material after the stress relief annealing, TS_(a) (MPa), will be TS_(a)<TS₀−X. For example, a cold rolled material, work hardened to 700 MPa at a final reduction ratio of 30%, is annealed for stress relief to less than 670 MPa to obtain a material with good bending workability.

(The Invention of Method of Manufacturing High-Strength Copper Alloy of Any of (1) to (5) Above Having Excellent Bending Workability, According to Any of (9) to (11) Above)

The methods according to (6) to (8) above [sic] are applicable to the manufacture of the high-strength copper alloy, especially phosphor bronze, of any of (1) to (5) above. The explanations already made of the preceding inventions generally apply to these methods as well.

(The Invention of Terminal Connectors According to (12) Above)

The inventions claimed above, in connection with solid-solution strengthened copper alloys, especially phosphor bronze type copper alloys, provide high-strength copper alloys having excellent bending workability and methods of manufacturing the same. The inventions apply to terminal connectors that require compactness in size, excellent bending workability, and high strength.

The contact portions of the terminal connectors undergo little deterioration in strength and bending workability upon plating before or after working, exhibiting the beneficial effects of the inventions.

WORKING EXAMPLES

The effects of embodiment of the present invention will now be explained in connection with various phosphor bronze products.

1) Example Series 1 Examples of the Inventions Defined in (1) to (3) Above

Phosphor bronze stocks of the compositions given in Table 1 were charcoal-coated in air, melted, and cast into ingots each measuring 100 mm wide, 40 mm thick, and 150 mm long.

The cast ingots were homogenized in an atmosphere of 75% N₂+25% H₂ at 700° C. for one hour, and the tin segregation layer formed on the surface was removed by means of a grinder.

Cold rolling and recrystallization annealing were then repeated a plurality of times each. In particular, the cold rolling reduction ratio before the final annealing, the final recrystallization annealing, and the final cold rolling reduction ratio were adjusted so that 0.2 mm thick sheets could be obtained.

The properties of the sheets thus obtained are shown in Table 1.

(Testing Procedures)

Tensile strength (TS: MPa) and 0.2% yield strength (YS: MPa) of a test specimen No. 13B (conforming to JIS Z 2201) sampled in the direction parallel to the rolling direction of each stock were found by a tensile test (JIS Z 2241).

Grain size is determined by the intercept method (JIS H 0501) which consists in counting the number of the grains completely sectioned along a predetermined length of segment, and finding the mean value of the cut lengths as the grain size. The standard deviation (σGS) is that of the grain size thus obtained. The sectional structure normal to the rolling direction as a scanning electron microscope (SEM) image is magnified 4,000 times, and each 50 μm-long line segment is divided by the number of points of intersections between the line and grain boundary to find the grain size. For the purposes of the present invention, the mean of the individual grain sizes so determined with 10 segments is deemed as the mean grain size (mGS) and the standard deviation of those grain sizes is deemed as the standard deviation (νGS).

Bending workability (r/t) is determined in the following way. Each test specimen, 10 mm wide and 100 mm long, is sampled in the transverse direction to the rolling direction and subjected to a W bend test (JIS H 3110) to various bend radii. The minimum bend radius ratio (r (bend radius)/t (thickness of the specimen)) is found at which a good outward appearance without fracture or orange peel is obtained at or above Rank C of the evaluation standards according to Japan Rolled Copper and Brass Association Technical Standards JBMAT307: 1999. (According to the evaluation standards, Rank A represents a product with no wrinkle; Rank B, slight wrinkle; Rank C, much wrinkle; Rank D, slight fracture; and Rank E, much fracture, Ranks A, B, C being evaluated as passable.) The axis of bending in the W bend test is parallel to the rolling direction.

TABLE 1 After anneal at 425° C. for 10,000 sec. 2.7 × exp Composition mGS σGS TS − YS 500 + 15 × Sn (0.0436 × Sn) TS (mass %) (μm) (μm) (MPa) (MPa) (μm) (MPa) r/t Example 1 Cu—4.2Sn—0.13P 4.9 0.8 7 563 3.2 556 0.5 of 2 Cu—6.2Sn—0.13P 4.0 0.7 15 593 3.6 630 0.5 Invention 3 Cu—8.0Sn—0.13P 3.9 0.6 4 620 3.8 733 2.0 4 Cu—10.0Sn—0.13P 3.5 0.6 22 650 4.2 783 2.0 5 Cu—4.2Sn—0.13P 2.3 0.6 5 563 3.2 600 0.5 6 Cu—6.2Sn—0.13P 2.5 0.7 11 593 3.6 652 0.5 7 Cu—8.0Sn—0.13P 1.5 0.4 4 620 3.8 753 2.0 8 Cu—10.0Sn—0.13P 1.0 0.3 17 650 4.2 848 3.5 Comparative 1 Cu—4.2Sn—0.13P 10 1.3 15 563 3.2 550 1.5 Example 2 Cu—6.2Sn—0.13P 13 2.0 20 593 3.6 625 1.5 3 Cu—8.0Sn—0.13P 14 1.5 8 620 3.8 728 3.0 4 Cu—10.0Sn—0.13P 12 2.5 30 650 4.2 790 4.0 Comparative A Cu—6.2Sn—0.13P 3.9 1.6 15 593 3.6 627 1.5 Example B Cu—8.0Sn—0.13P 4.2 0.7 104 620 3.8 715 3.0 C Cu—8.0Sn—0.13P 15 2.0 117 620 3.8 718 3.5 Inv. D Cu—8.0Sn—0.13P 1.7 0.4 60 620 3.8 684 1.0 Com. E Cu—8.0Sn—0.13P 14 2.5 64 620 3.8 681 2.0

Table 1 shows Examples 1 to 8 of the present invention and Comparative Examples 1-4 of the prior art materials. Also, in order to explain the effects of the present invention, additional examples A to E are shown as classified by way of convenience according to varied parameters (Com. stands for Comparative Example and Inv. stands for inventive examples).

A comparison between Comparative Examples 1 to 4 of the prior art materials and Examples 1 to 4 and D of the present invention shows that, while the composition and strength are the same, Examples 1 to 4 and D of the present invention are improved in bending workability with lower r/t values.

Example D of the invention is an example of a high TS-YS value in (1) above (or an example aimed at clarifying the definition of TS-YS≦80, indicating that its bending workability is improved over Comparative Example E of about the same strength).

Examples 5 to 8 of the invention are examples in which the grain sizes of Examples 1 to 4, respectively, were made finer. They show that the strength is improved, the r/t is the same or smaller, and the bending workability is enhanced through the adjustment of the grain size according to the tin concentration in conformity with mGS<2.7×exp(0.0436×Sn).

Comparative Example A is inferior in bending workability to Example 2 of the invention because its mGS satisfies the requirement of (1) above but its σGS does not.

Comparative Example B is an example that satisfies the requirement of (1) above as to both mGS and σGS but fails to satisfy the TS-YS requirement. Although the grains after the annealing are fine, the high TS-YS reduces strength and makes the material about equal to the conventional material C in both strength and bending workability, with no indication of improvement.

Comparative Example C is mentioned by way of comparison with Comparative Example B.

Comparative Example E is by way of comparison with Comparative Example D.

(2) Example Series 2 Examples Verifying the Inventions Defined in (4) and (5) Above

Test specimens of compositions based on phosphor bronze constituents with the addition of iron, nickel or the like were made following the procedure of Example series 1.

The state of dispersion of precipitation and crystallization products of the compounds formed by the particular kinds of elements added was adjusted by appropriate choice of homogeneous annealing conditions for the cast ingots.

Recrystallization annealing was adjusted under the observation of the residual state of coarse precipitation and crystallization products and the growth of the precipitation products, along with the adjustment of the grains.

The numbers of particles of the precipitation and recrystallization products 0.1 μm or larger across were analyzed and observed using an energy-distribution analyzer of a field-emission scanning electron microscope.

Table 2 summarizes the results.

TABLE 2 After anneal at 425° C. for 10,000 sec. No. of TS − 2.7 × exp Composition mGS σGS sectnl YS 500 + 15 × Sn (0.0436 × Sn) TS (mass %) (μm) (μm) partcl * (MPa) (MPa) (μm) (MPa) r/t Inventive 9 Cu—4.1Sn—0.13P—0.2Fe—0.5Zn 3.0 0.4 30 4 562 3.2 586 0.5 Example 10 Cu—6.1Sn—0.13P—0.5Ni—0.5Fe 4.3 0.6 55 13 592 3.5 644 0.5 11 Cu—8.2Sn—0.13P—0.5Mg 4.4 0.6 48 4 623 3.9 756 1.5 12 Cu—10.2Sn—0.13P—0.8Ni—0.4Si 4.7 0.7 67 20 653 4.2 783 2.0 13 Cu—4.1Sn—0.13P—0.2Fe—0.5Zn 2.2 0.4 455 4 562 3.2 608 0.5 14 Cu—6.1Sn—0.13P—0.5Ni—0.5Fe 2.5 0.4 160 10 592 3.5 687 0.5 15 Cu—8.2Sn—0.13P—0.5Mg 1.2 0.3 220 4 623 3.9 789 2.0 16 CU—10.2Sn—0.13P—0.8Ni—0.8Si 0.9 0.2 240 16 653 4.2 855 3.5 Comparative 1 Cu—4.ZSn—0.13P 10 1.3 — 15 563 3.2 550 1.5 Example 2 Cu—6.2Sn—0.13P 13 2.0 — 20 593 3.6 625 1.5 3 Cu—8.0Sn—0.13P 14 1.5 — 8 620 3.8 728 3.0 4 Cu—10.0Sn—0.13P 12 2.5 — 30 650 4.2 790 4.0 Inventive A Cu—6.1Sn—0.13P—0.1Cr—0.1Ti 1.6 0.3 420 14 592 3.5 701 1.0 Example B Cu—6.1Sn—0.13P—0.2Cr—0.1Zr 1.3 0.2 530 20 592 3.5 711 1.0 C Cu—6.1Sn—0.13P—0.03Al—0.3Mn 2.5 0.7 160 12 592 3.5 669 0.5 D Cu—6.1Sn—0.13P—0.03Ag—0.2In 2.4 0.6 150 8 592 3.5 664 0.5 E CU—6.1Sn—0.13P—0.1Be—0.03Ca 2.3 0.4 200 11 592 3.5 672 0.5 F Cu—6.1Sn—0.13P—0.1Be—0.2Ti 2.0 0.3 260 14 592 3.5 690 0.5 G Cu—6.1Sn—0.13P—0.03Y—0.1Nb 2.0 0.4 240 14 592 3.5 685 0.5 Comp H Cu—6.1Sn—0.13P—2.3Fe—0.4Zn 1.4 0.4 540 15 592 3.5 762 4.5 * Number of particles 0.1 μm or larger per millimeter square of a section cut in parallel with the rolling direction.

It is obvious from a comparison with the Cu—Sn—P alloys of the present invention listed in Table 1 that the addition of minor amounts of other elements makes the σGS smaller and permits further grain refinement in a stable manner and that the consequent dispersion of the particles consisting of those elements adds strength and enhances the bending workability.

Similar beneficial effects were confirmed with the alloys that contained Cr, Ti, Zr, Nb, Al, Ag, Be, Ca, Y, Mn, or/and In. Examples of those alloys are shown too as A to H in Table 2 (wherein Com. stands for Comparative Example and Inv. stands for inventive example).

Comparative Example H is an example in which the total amount of the auxiliary constituents exceeded 2.0 mass % and the resulting alloy was inferior in bending workability.

(3) Example Series 3 Examples Verifying the Inventions Defined in (6), (7), (9), and (10) Above

The compositions of Examples 17 to 20 of the present invention correspond, respectively, to those of Examples 1 to 4 in Table 1. Comparative Examples 5 to 8 are examples of conventional materials. In order to demonstrate the effects of the present invention, additional Examples A to F of altered parameters are shown as classified separately by way of convenience (Com. stands for Comparative Example and Inv. for inventive example). The testing procedure was generally in conformity with that used in Example series 1. Table 3 summarizes the results.

TABLE 3 Reduction rate After recrystallization of cold rolling before anneal Reduction rate Composition recrystallization mGS σGS of final cold TS (mass %) anneal (%) (μm) (μm) rolling (%) (MPa) r/t Inventive 17 Cu—4.2Sn—0.13P 48 2.0 1.0 30 623 1.5 Example 18 Cu—6.2Sn—0.13P 50 1.8 1.2 25 710 1.0 19 Cu—8.0Sn—0.13P 50 1.6 1.0 25 746 1.5 20 Cu—10.0Sn—0.13P 60 1.1 0.7 30 901 4.0 Comparative 5 Cu—4.2Sn—0.13P 40 6.0 2.1 35 602 2.0 Example 6 Cu—6.2Sn—0.13P 40 8.2 2.3 30 652 1.0 7 Cu—8.0Sn—0.13P 44 5.0 2.2 25 680 2.0 8 Cu—10.0Sn—0.13P 40 4.2 2.1 30 805 3.5 Invention A Cu—8.0Sn—0.13P 50 2.6 1.2 25 718 1.5 B Cu—8.0Sn—0.13P 50 2.6 1.3 15 626 0 Comparative C Cu—8.0Sn—0.13P 40 2.8 2.2 25 710 2.0 Example D Cu—8.0Sn—0.13P 50 2.8 2.1 25 715 2.0 E Cu—8.0Sn—0.13P 50 2.7 1.3 5 550 0 F Cu—8.0Sn—0.13P 50 5.0 2.3 10 560 0

Comparative Examples 5 to 8 are examples of conventional materials, with the reduction ratio of the cold rolling before the final annealing and the mean grain size at the final annealing being both outside the ranges specified under the invention. Specimens of Examples 17 to 20 according to the present invention showed greater strength, lower r/t, and better bending workability than the conventional ones of Comparative Examples 5 to 8.

Example A of the invention meets the grain size requirement of (6) above but not the requirement of (7) above in that the size after the recrystallization annealing in Example 19 of the invention was increased to 2.6. Example 19 in which the grain size was smaller showed somewhat greater strength.

Example B of the invention is an example in which the reduction ratio of the final cold rolling satisfied the requirement of (6) above but was too low to meet the requirement of (7) above. The bending workability was good in inverse proportion to the strength.

Comparative Example C was inferior in bending workability to Example A of the invention since the reduction ratio of the cold rolling prior to recrystallization was low and, although the mGS was made smaller by the recrystallization annealing, the grains obtained was not fine or homogeneous, with wide scatter of grain size (σGS).

Comparative Example D is an example that met both the rolling reduction ratio and mGS requirements of (6) and (7) above but failed to meet the σGS requirement because of a unsuitable thermal condition during the recrystallization annealing. The bending workability was unsatisfactory as in Comparative Example C.

Comparative Example E is an example of a low reduction ratio of final cold rolling. The strength was low, about the level of the conventional material of Comparative Example F, and indicated no ameliorating effect.

Comparative Example F is, as noted above, a conventional example (with about the same TS and the same r/t as E)

(4) Example Series 4 Investigations about the Effect of Stress Relief Annealing According to (8) and (11) Above

Referring to Table 4, Examples 21 to 28 of the present invention, as noted also in the table, correspond, respectively, to Examples 2, 3, 4, 7, 8, 15, 16, and 20 of the invention mentioned already, and Comparative Examples 9 to 12 (of conventional materials) correspond to the above-mentioned Comparative Examples 3, 4, 7, and 8. Comparative Examples A and B, which are cited as examples of low TS values decreased by the stress relief annealing, correspond to Examples 16 and 20 of the invention.

Test specimens of these materials were annealed for stress relief after varied conditions of different reduction ratio of final cold rolling and then were evaluated for their properties. The amounts of decrease in the tensile strength (TS) due to the stress relief annealing are also given.

TABLE 4 Reduction percentage of final cold rolling of stress-relief annealed TS reduced by stress TS specimen (%) relief annealing (MPa) (MPa) r/t Example 21 Inventive Example 2 (25) 60 570 0 of the 22 Inventive Example 3 (25) 81 652 0 Invention 23 Inventive Example 4 (25) 35 748 1.5 24 Inventive Example 7 (25) 30 723 1.5 25 Inventive Example 8 (30) 44 804 2.5 26 Inventive Example 15 (25) 29 760 2.0 27 Inventive Example 16 (30) 57 798 2.5 28 Inventive Example 20 (35) 52 849 3.0 Comparative A Inventive Example 16 (30) 14 841 3.0 Example B Inventive Example 20 (25) 15 886 3.5 9 Comparative Ex. 3 (30) 30 698 2.5 10 Comparative Ex. 4 (30) 84 706 3.0 11 Comparative Ex. 7 (25) 30 650 1.5 12 Comparative Ex. 8 (30) 82 762 3.0

Example 21 of the invention is a material with a tin concentration of 6.2 mass %. Its tensile strength (TS) was 570 MPa and bending workability (r/t) was 0.

Examples 22, 24, and 26 of the present invention and Comparative Examples 9 and 11 of conventional materials all ranged in tin concentration from 8.0 to 8.2 mass %. However, the examples of the invention exhibited tensile strength (TS) values from 652 to 760 MPa and bending workability (r/t) from 0 to 2.0, whereas the comparative examples had tensile strength from 650 to 698 MPa and r/t from 1.5 to 2.5, indicating that the materials according to the invention had greater strength and better bending workability.

Examples 23, 25, 27, and 28 of the invention and Comparative Examples 10 and 12 had about the same tin concentrations from 10.0 to 10.2 mass %. However, the examples of the invention showed tensile strength (TS) values from 748 to 849 MPa and bending workability (r/t) from 1.5 to 3.0, whereas the comparative examples had tensile strength from 706 to 762 MPa and r/t of 3.0, indicating again the superiority of the materials according to the invention in both strength and bending workability.

Comparative Examples A and B had tensile strength (TS) from 841 to 886 MPa, but the amounts of the TS reduced by stress relief annealing were small and the bending workability (r/t) values were not much improved, in the range from 3.0 to 3.5.

As can be seen from the foregoing, the materials that have been stress relief annealed in accordance with the present invention are undoubtedly improved in strength and bending workability over the conventional materials of the comparative examples. The strength being about the same, the inventive materials are remarkably improved in bending workability over the comparative materials and, the bending workability being the same, the strength is likewise greatly improved.

EFFECTS OF THE INVENTION

Examples of the present invention thus demonstrate that the invention makes it possible to impart great strength to copper alloys, especially phosphor bronze type alloys, without adversely affecting their bending workability. They also show that the invention brings improvements in the properties required of copper alloys for use in terminal connectors of electronic parts.

The invention further permits the advance of the high-tin phosphor bronze (Cu-10mass % Sn—P: CDA52400) in the market of high-strength copper alloys that has hitherto denied its access on the ground of poor bending workability and has been dominated by beryllium coppers and the like. 

1. A high-strength copper alloy having excellent bending workability that is a finally cold rolled copper alloy with a tensile strength and 0.2% yield strength different by not more than 80 MPa, the alloy having the mean grain size (mGS) thereof after annealing at 425° C. for 10,000 seconds not more than 5 μm and the standard deviation of the mean grain size (σGS) not more than ⅓×mGS, wherein the alloy consists essentially of from 1 to 11 mass % Sn, from 0.03 to 0.35 mass % P, and the balance Cu and unavoidable impurities.
 2. A high-strength copper alloy having excellent bending workability according to claim 1 with a tensile strength termed TS_(Sn) (MPa) being TS_(Sn)>500+15×Sn (Sn: tin concentration (mass %)).
 3. A high-strength copper alloy having excellent bending workability according to claim 1, the alloy having characteristics such that the mean grain size (mGS (μm)) thereof after annealing at 425° C. for 10,000 seconds is mGS<2.7×exp (0.0436×Sn (Sn: tin concentration (mass %)).
 4. A high-strength copper alloy having excellent bending workability, wherein the high-strength copper alloy is a phosphor bronze which consists essentially of from 1 to 11 mass % Sn, from 0.03 to 0.35 mass % P, from 0.05 to 1.0 mass %, in total, of one, two, or more selected from among Fe, Ni, Mg, Si, Zn, Cr, Ti, Zr, Nb, Al, Ag, Be, Ca, Y, Mn, and In, and the balance Cu and unavoidable impurities, and is a finally cold rolled copper alloy with a tensile strength and 0.2% yield strength different by not more than 80 MPa, the alloy having the mean grain size (mGS) thereof after annealing at 425° C. for 10,000 seconds not more than 5 μm and the standard deviation of the mean grain size (σGS) not more than ⅓×mGS.
 5. A high-strength copper alloy having excellent bending workability according to claim 4, wherein the alloy is a phosphor bronze with particles that consist essentially of precipitation or crystallization products of the alloying metals, 0.1 μm or more in diameter, being present in a number of not fewer than 100 per square millimeter of a cross section cut in parallel to the rolling direction.
 6. A method of manufacturing high-strength copper alloy having excellent bending workability comprising the steps of cold rolling to a reduction percentage of at least 45%, final annealing to the extent that the mean grain size (mGS) is not more than 3 μm and the standard deviation of the mean grain size (σGS) is not more than 2 μm, and final cold rolling to a reduction percentage of from 10 to 45%.
 7. A method according to claim 6 of manufacturing high-strength copper alloy having excellent bending workability comprising the steps of final annealing to the extent that the mean grain size (mGS) is not more than 2 μm and the standard deviation of the mean grain size (σGS) is not more than 1 μm, and final cold rolling to a reduction percentage of from 20 to 70%.
 8. A method of manufacturing high-strength copper alloy having excellent bending workability according to claim 6 comprising stress relief annealing of the cold rolled material that has been finally cold rolled to a reduction ratio X (%) and has a tensile strength of TS₀ (MPa), until the tensile strength TS_(a) (MPa) after the annealing is TS_(a)<TS₀−X.
 9. A method according to claim 6 of manufacturing high-strength copper alloy having excellent bending workability defined in claim
 1. 10. A method according to claim 7 of manufacturing high-strength copper alloy having excellent bending workability defined in claim
 1. 11. A method according to claim 7 of manufacturing high-strength copper alloy having excellent bending workability defined in claim
 1. 12. A terminal connector using the high-strength copper alloys having excellent bending workability according to claim
 1. 13. A high-strength copper alloy having excellent bending workability according to claim 4, with a tensile strength termed TS_(Sn) (MPa) being TS_(Sn)>500+15×Sn (Sn: tin concentration (mass %)).
 14. A high-strength copper alloy having excellent bending workability according to claim 4, having mGS<2.7×exp(0.0436×Sn) where mGS is the mean grain size in μm of the alloy after annealing at 425° C. for 10,000 seconds, and Sn is the tin concentration in mass %.
 15. A terminal connector using the high-strength copper alloy having excellent bending workability according to claim
 13. 